The myoepithelial cells of the proventriculus of the marine polychaete worm Syllis spongiphila consists of only one or two sarcomeres which may reach 4o microns in length. The cells can be isolated by dissection. Electrophysiological experiments are planned to correlate membrane potential and length of cells in situ and to study the development of tension in single, isolated cells. Techniques will be developed to permit recording of tension, membrane potential and overlap of thick and thin filaments. The myoepithelial cells undergo regenerative spikes which appear to depend on an influx of calcium ions. Manganese, which competes with calcium in many preparations, can replace calcium in generating spikes in the Syllis myoepithelial cells; a similar characteristic is exhibited by certain mammalian myocardial cells. Studies on the permeability changes exhibited by Syllis myoepithelial cells are planned, both to extend the understanding of calcium permeability in general and to determine whether the myoepithelial cells may be used as a model for certain studies of cardiac membrane excitability. The myoepithelial cells exhibit a sparse sarcoplasmic reticulum. To test if exogenous calcium is required for the intiation of contraction, membrane potential and contractile activity will be monitored while pharmacological agents are used to block calcium current or to stimulate release of calcium from internal stores.